select all options in html select dynamically

Answer 1

Try this one UPDATED!

HTML

<select name="sortedby" multiple="multiple" id="sortedby" onchange="putvalue()">
    <option value="start">Start time</option>
    <option value="thread_id">Thread Id</option>
    <option value="user_name">Username</option>
</select>

<select name="sortedby_target" id="sortedby_target" multiple="multiple" ></select>

SCRIPT

function putvalue(){
    $('option[value="' + $('#sortedby_target').val() + '"]').prop('selected','');
    $("#sortedby_target").append('<option value="' + $('#sortedby').val() + '" selected>' + $('#sortedby').val() + '</option>');
    $("#sortedby option[value=" + $('#sortedby').val() + "]").remove();
}

I apologized for my first solution hopefully this helps.

If you want to be selected all the selected items, remove this line

$('option[value="' + $('#sortedby_target').val() + '"]').prop('selected','');